What advantages would this give over plain Debian or similar? I'm a total noob, so I'd love something that might help me get a little more out of my little netbook 'server'.
dietPi is in fact Debian, with extra scripts to install/remove software. They also thinned it way down, so you get a working system with the bare essentials.
If you're all set up on Debian, I don't see the advantage of switching to another flavor of Debian, unless you have a low powered machine (low specs, not much RAM).